An image display system has been proposed that has a plurality of image transmission devices and an image display device that is communicably connected to these image transmission devices wherein the image display device displays image data that have been transmitted from each image transmission device. Patent Document 1 describes related art of such an image display system.
The image display device is provided with a display unit in which the display screen is partitioned into a plurality of divided display areas that correspond to each of the image transmission devices, and image data that are received from each image transmission device are displayed in a corresponding divided display area. The image display device transmits a transmission request of the image data to each image transmission device, and in accordance with the transmission request, each image transmission device transmits the image data to the image display device. The image display device transmits the transmission requests of the image data of the next frame to the image transmission devices at the timing of receiving a one-frame portion of image data.
The above-described image display system is used in, for example, a conference. In the form of use of, for example, a conference, a plurality of participants typically connect information processing terminals (image transmission devices) such as their own notebook personal computers or mobile terminals to a projector (image display device) by way of a network such as a LAN (Local Area Network). Each information processing terminal transmits data of images that are displayed on their own terminals to the projector in accordance with transmission requests from the projector. In some cases, the processing capability (CPU processing speed) of the information processing terminals differs for each participant.
However, because each image transmission device simultaneously sends image data to the image display device in the above-described image display system, pressure is placed on the network bandwidth, with the result that the communication speed decreases. In particular, using a network having comparatively narrow bandwidth in a mode of use such as a conference places extreme pressure upon the network bandwidth due to simultaneous transmission of image data by each image transmission device. Here, the network bandwidth indicates the amount of data transmitted per unit time.
The method described in Patent Document 2 is one method of preventing decrease of the communication speed of a network. In this method, an image transmission device compares two images that continue in time for an image that is displayed on the screen in a time series, extracts the portion in which change occurs, i.e., the differences of the images, and transmits to the image display device compressed data in which the difference data are compressed. If this method is applied in the above-described image display system, the amount of transmitted data per single frame that is transmitted by each image transmission device can be reduced, whereby decrease of the communication speed that occurs due to pressure upon the network bandwidth can be mitigated. The image transmission device is able to accumulate data that indicate the differences between a plurality of images that continue in time and then compress and transmit the accumulated differential data in accordance with transmission requests.
However, in the case described above, the amount of the differences changes in accordance with the range and content of the changes of the images that are displayed in the image transmission device, and together with this change, the amount of transmission of compressed data that are transmitted to the image display device also changes. When the amount of compressed data transmitted changes, the data transmission time per single frame also changes, with the result that the image updating of the divided display areas on the screen of the image display device becomes irregular. As a result, the updating of the image that is displayed in the divided display areas on the screen of the image display device is no longer in synchronization with the change of the image that is displayed on the image display device, and the users of the image transmission devices will therefore perceive unnatural display when viewing the display image on the image transmission device side and the display image on the image display device side.
In order to suppress this unnatural display, the image display device transmits a transmission request to each image transmission device at a fixed transmission interval such that the compressed data from each image transmission device can be received at a fixed period.